Schaefer, Richard T, author
New York, 2003
Buku Teks
Schaefer, Richard T, author
McGraw-Hill, 1995
Buku Teks
McGraw-Hill, 2003
Buku Teks
Hunt, Elgin F., author
MacMillan, 1987
Buku Teks
Sumner, William Graham
New Haven Yale University Press, 1950
Buku Teks